Method for producing plate-shaped components or combinations of components

ABSTRACT

Method for producing a plate-shaped component. This involves the preparation of a first suspension comprising an alcohol as the dispersing medium and a base material for the plate. A second suspension is prepared which comprises binder, water as the gelling agent and an alcohol as the dispersion medium, possibly with the addition of Li/K or Li/Na carbonate. These two suspensions are mixed together, possibly with the addition of fibers. After mixing, a plate-shaped component is fabricated with the aid of the “tape casting” technique.

BACKGROUND OF THE INVENTION

The present invention relates to a method in accordance with thepreamble of claim 1.

Such a method is disclosed in the PCT application Wo 96/08050.

Various proposals exist in the prior art for producing, for example, amatrix plate in a fuel cell.

The oldest proposal comprises the so-called hot press technique. In acomparatively complicated manner this technique yields comparativelythick plates. The result of this, on the one hand, is an increase inproduction costs and, on the other hand, a reduction in the efficiencyof the cell.

An alternative for producing matrix plates is a process which is similarto the papermaking process. This involves mixing pulp comprising abinder, water and the supporting materials of the matrix. The matrixmaterial then adheres to the pulp, giving rise to floccules. Via thefiltration process the floccules are separated from the water material,and the binder is removed by subsequent baking. This process isattractive since the suspension medium used is inexpensive,environment-friendly water. A drawback is its controllability and, inparticular, the pore size and the pore size distribution. It did notprove possible to fabricate matrix plates reproducibly in this way on acommercially acceptable scale.

The third most commonly used technique is the so-called “tape casting”.

This involves a suspension in which a binder, fibres and the matrixmaterial are present being cast onto a flat table and the suspensionmedium present therein being evaporated. A green matrix plate is thenproduced which can either be sintered before being placed into a fuelcell, after which the electrolyte can be introduced, or is placeddirectly into the fuel cell. In the latter case the electrolyte isintroduced in some other way.

When the suspension was being prepared it was not uncommon to employvacuum conditions in order thus to regulate the viscosity of the fluid.

Such a method is described, for example, in the article “Review ofCarbonate Fuel Cell Matrix and Electrolyte” by H. C. Maru et al. inProceedings of 2nd Symposium on MCFC Technology, Vol. 90-16, pp.121-136. The binder described therein was dissolved in a mixture ofxylenes and ethanol. When the suspension is heated there is the problem,in particular, of removing xylene. It is no longer permissible for asolvent such as xylene to be discharged into the environment, andrecycling leads to a considerable increase in costs.

A similar consideration applies to the use of acrylate as a binder whichis dissolved in a mixture of acetone, dichloromethane and petroleumnaphtha.

In the case of the method described in the abovementioned PCTapplication 96/08050 it was found that in the process of forming thatsuspension, solid particles will agglomerate, resulting in a decrease inthe strength of the tape. This is caused by uneven dispersion of theadditions and a pore structure which is not well defined. Moreover theintroduction of additions such as fibre strengtheners, dissolutionretardants and carbonate proved particularly difficult. Uponintroduction of such substances agglomeration has likewise beenobserved, which counteracts the strength, on the one hand, and thedispersal of these substances, on the other hand.

Moreover it was found that this production gives rise to rejects andwaste which cannot be put to any further use.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid these drawbacks.

This object is achieved by means of the characterizing measures of claim1.

By preparing two ancillary suspensions and then mixing these, it wasfound, surprisingly, that agglomeration of solids does not occur.Moreover, the above-described additions can readily be added to thesuspension in question without the risk of agglomeration orinhomogeneities, as the case may be.

Should one of the two suspensions not meet the set requirements, it canbe readily recycled.

The above-described method requires less aftertreatment (such asrolling) and makes it possible for production rejects or waste to beused directly a starting material. In the process, production wasteafter “tape casting” can be reused by the addition of water/alcoholmixture; the gelled binder present is water-soluble. In the case ofother binder systems such as acrylates this is not possible. In thiscontext it must be ensured that the properties of the plate-shapedcomponent obtained are uniform and adjustable, in a simple manner, as afunction of the requirements imposed thereon, such as pore size andthickness.

This object is achieved in the case of an above-described method by afirst suspension being prepared, comprising an alcohol as the dispersionmedium and the base material, by a second suspension being prepared,comprising the binder and an alcohol as the dispersion medium m andwater as the gelling medium, and these two separately preparedsuspensions being mixed with one another before being cast.

By using a different dispersant gelling agent/binder system, the bindersuspension and the suspension which comprises the material of thecomponent being fabricated it proved possible to dispense with organicsolvents other than alcohols.

In contrast to the view held in the prior art (see for example thearticle “Physical Property Optimization of Lithium Aluminates forFabrication of Molten Carbonate Fuel Cell Matrices” by Patrick M. Brownin Fuel Cell Seminar 1990, pp. 289-293) the addition of water needs notto have a negative effect on the results obtained with a suspensionprepared in this manner which is then cast.

It was found that a component which, in the abovementioned manner, wasfabricated from suspensions on a water/alcohol basis with the aid of the“tape casting” technique, can be held in intermediate storage forconsiderably longer, be produced more cheaply and results in lesspollution of the environment than a plate which has been fabricated withthe aid of an acrylate binder system.

It was found, moreover, that if this dispersant/gelling agent/binder isused, production rejects can be used directly as a starting material.

Employing a dispersing/gelling agent/binder system on the basis ofwater-alcohol mixtures makes it possible to use suspensions within awide viscosity range of 500-3500 cPs, preferably 1000-20000 cPs of thevarious suspensions. As a result it is no longer necessary to employ avacuum for precise regulation of the viscosity of the suspension.

The amount of water added to the gelling binder suspension is preferablyat least 10-25 times the weight of the binder.

If a surfactant is added to prevent coagulation, this is preferablyintroduced into the first above-described suspension.

The first suspension may comprise any of the granular materials knownfrom the prior art. Examples to be mentioned are: nickel, nickel oxide,nickel aluminum, both alloyed and intermetallic, lithium cobaltate,chromium, lithium aluminate and aluminium oxide, and also other metalsor metal oxides in the form of powders or fibres. The solid may alsoconsist of mixtures of the abovementioned substances.

The use in an MCFC cell and more in particular as a matrix plateinvolves, as generally known in the prior art, (γ-)lithium aluminate. Byemploying the “tape casting” technique it is possible to cheaplyfabricate comparatively thin plate-shaped components which after dryingcan be used directly in the cell. The low thickness can result inoptimum capacity of e.g. a fuel cell.

It is also possible for the plate-shaped component fabricated by meansof the “tape casting” technique then to be baked or sintered and then toadd an electrolyte to the said component in some way known from theprior art, if the plate-shaped component is employed as an electrode ormatrix in an electrochemical cell.

Such an addition may comprise impregnation with e.g. an electrolytematerial. It is also possible for a layer of solid carbonate to beplaced into a fuel cell between, for example, two plates and for thefuel cell to be heated subsequently, as a result of which the carbonatematerial, which then melts, is absorbed into the plates by capillaryaction.

The latter method faces difficulties, however, if a comparatively tallfuel cell stack, i.e. an electrochemical pile comprising a large numberof cells has to be fabricated. The risk of tilting when the carbonate ismelted and possibly incorrect assembly is increased by the increase inthe number of components.

The first method described, where after baking or sintering of thecomponent a suspension of the electrolyte is added, has the drawbackthat renewed heating is required afterwards. Such a method iscomparatively complicated and increases the costs.

Using the above-described method according to the invention it provedpossible, however, to add an electrolyte suspension to the bindersuspension. If the carbonate material is employed, this again does notrequire the use of organic solvents other than alcohols.

As well as with material which subsequently provides the electrolyte,the second suspension may also be admixed with a material which changesor regulates the degree of acidity of the electrolyte. Thus thedissolution rate of the cathode in the electrolyte of e.g. an MCFC cellcan be controlled. Examples of such an agent are lithium ferrite andsalts of alkaline earth metals.

By integrating the absorption of the electrolyte into the production ofthe plate-shaped component it is possible to achieve considerablesimplification of the various process steps, while in addition thedrawbacks deriving from the use of the large number of components in thecase of carbonate material being fused in situ during the start-up of afuel cell are avoided.

The electrolyte suspension will be chosen as a function of theelectrolyte to be obtained. If the electrolyte comprises carbonatematerial, as in the case of an MCFC cell, preference is given to the useof a mixture of Li₂CO₃, Na₂CO₃ and K₂CO₃ in any desired ratio.

The above-described first and second suspension may be admixed withvarious aids generally used in the prior art. Thus it is possible to addan anti-foaming agent and/or release agent and/or plasticizer.

The alcohols used may likewise comprise all the alcohols known from theprior art, but in view of feasibility and cost price preference is givento ethanol.

It is possible for fibres to be incorporated in the first suspensionwhen the matrix plates are being fabricated, to strengthen thecomponent.

The fibres used may, apart from the aluminium oxide mentioned, compriseany fibre known from the prior art. These may or may not be groundbefore use.

However, it is also possible to prepare a separate third suspensionconsisting of a dispersion of fibres and an alcohol. The fibres used maycomprise, for example, aluminium oxide materials or lithium aluminatefibres, and the alcohol may comprise ethanol, as specified above.

If the abovementioned method is used for producing components of an MCFCfuel cell, and more in particular the cathode thereof, it is advisableto add agents during the preparation, which retard dissolution of thecathode and more in particular of the NiO therein. Worth mentioning, forexample, is an alkaline earth metal carbonate. One example is a mixturecomprising roughly equal proportions of lithium and sodium carbonate,which is admixed with small amounts of calcium, strontium and/or bariumcarbonate.

The invention is explained below in more detail with reference to theflow diagram according to the only figure and to examples.

In the figure, the above-described method is explained in a flowdiagram.

This shows that for the purpose of e.g. the production of the matrixplate during a first step a mixture comprising ethanol, Dolapix (ananticoagulant) and lithium aluminate is ground and homogenized. Theethanol may be denatured, i.e. contain about 4% of methanol.

Grinding can be carried out batchwise or continuously. In the case ofbatchwise grinding a ball mill, for example, is preferably used, and inthe case of continuous grinding an attritor, for example.

It will be understood that instead of lithium aluminate other materialscan be used which are generally known from the prior art.

BRIEF DESCRIPTION OF THE INVENTION

In addition to the preparation of the first suspension, a second andthird suspension are also prepared.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The second suspension comprises ethanol, a binder and water. This binderis dispersed and gelled. The second suspension may additionally beadmixed, as indicated by dotted lines, with lithium/potassium orlithium/sodium carbonate or any other agent which provides theelectrolyte. The third suspension comprises ethanol and fibres. Then thethree suspensions thus obtained are mixed, with the addition of TBP(tributyl phosphate) or some other anti-foaming agent and release agentand PEG (polyethyleneglycol) or any other plasticizer.

It will be understood that the various grinding and homogenization stepsor gelling steps, respectively, are essential and that the formation oflumps or agglomerates must be precluded as far as possible.

The mixture obtained is then mixed again and homogenized, screened,freed from air and then applied by means of the “tape casting”technique, followed by drying.

De-aeration is important since there is the possibility of air bubblesbeing incorporated into the suspension during the various processingsteps. Should no other steps be taken there is the risk of so-calledpinholes forming in the component.

EXAMPLE I

To prepare 12 liters of casting suspension, to be used in the “tapecasting” technique, three different suspensions were prepared.

The first suspension consisted of 3,000 g of ethanol, 70 g of Dolapix ET85 and 2800 g of LiAlO₂ powder. Dolapix is produced by SERVA inHeidelberg, Germany. To prepare the first suspension, ethanol andDolapix were poured into a stock vessel, followed by the addition, withcontinuous mixing, of the lithium aluminate powder, and the suspensionobtained was then ground in an attritor with the aid of apumped-circulation system. The grinding time was about 4 hours, afterwhich an average desired particle size of 2 μm was obtained.

To prepare the second suspension, 1500 g of ethanol were weighed, aswere 140 g of methylcellulose as the binder. The methylcellulose wasdispersed in the ethanol over a period of 5-10 minutes, after which 1820g of demineralized water were added, followed by stirring for at least60 minutes.

The third suspension was prepared by weighing 325 g of Al₂O₃ fibres and900 g of methylated ethanol.

Aluminium oxide fibres were added to the ethanol with stirring. Thestirring time was 30 minutes.

Then these three suspensions were mixed together with stirring. Thestirring time was 10 minutes, followed by the addition of 60 g of TBPand 820 g of PEG. TBP and PEG are available from Merck under the ordernumbers 818604 and 817003-50003, respectively. The suspension ultimatelyobtained was screened through a 500 μm screen and then de-aerated in aclosed vessel by the suspension being rolled slowly for 60 minutes on aroller frame. Then the suspension was cast with the aid of the “tapecasting” technique. This produced a matrix plate which was not providedwith electrolyte.

EXAMPLE II

To fabricate an anode, the following ingredients were added successivelyin a 6.5 liter vessel with constant stirring at 320 rpm: 2340 g ofethanol (5% methanol) and 65 g of methylcellulose. After 1 minute'sstirring, 1287 g of demineralized water was added, and after continuedstirring for 10 minutes, 39.0 g of Tween 20 and 39.0 g of TBP. TBP canbe obtained from Serva as mentioned above. The methylcellulose used wasgrade “MC-A4C” available from Dow Chemical.

The above-described mixture was mixed well and then, via a vibratinghopper which moved at a frequency of 6.5 Hz, admixed with 520 g of fineCr. This Cr powder is available from Johnson Matthey. During theaddition stirring proceeded at 340 rpm. Then, at a rotational speed of360 rpm, 4680 g of Ni 287 from Inco Europe Limited, London were added.

Stirring then continued for 55 minutes at 380 rpm, after which 260 g ofethanol (5% methanol) and 143 g of demineralized water were added.Stirring then continued for 5 minutes, after which the suspension wasrolled for 60 minutes in a closed vessel at 60 rpm. The mixture thusobtained was used for the “tape casting” technique, of course afterviscosity and temperature had been measured. This was followed bydrying, and the dried tape was then cut to size.

EXAMPLE III

The same method as in Example II was followed for the fabrication of thecathode, except that 2800 g of ethanol per 71 g of methylcellulose wereadded. After the said stirring for 1 minute, 1633 g of demineralizedwater were added, and after the mixture thus produced had been stirred,47.0 g of Tween 20 and 47.0 g of TBP were added.

The vibrating hopper was used, while stirring took place with increasingspeed, to add 3500 g of Ni 255 (available from Inco Europe Limited).After the said stirring for 30 minutes at 380 rpm, the suspension waspoured into a 6.5 1 vessel, followed by rolling for 60 minutes at 60rpm. Then the green cathode was prepared with the aid of the “tapecasting” technique.

Although the invention has been described above with reference to apreferred embodiment, it will be understood that this can be modified innumerous ways without moving outside the scope of the present inventionas described in the appended claims.

What is claimed is:
 1. A method for producing a plate-shaped componentelectrode or matrix plate of a fuel cell used at high temperature,comprising the preparation of a suspension comprising a base materialfor the plates, fibres, binders/gelling agents and an alcohol, whichsuspension is cast by means of a tape casting technique, after which thealcohol is removed from the suspension, a suspension being preparedwhich comprises an alcohol as a dispersion medium, the base material andthe binder, wherein the preparation of the suspension comprises:preparing a first suspension comprising an alcohol or dispersion mediumand the base material; preparing a second suspension comprising thebinder and a water/alcohol as dispersant/gelling agent, and mixing saidfirst and second suspensions.
 2. A method according to claim 1,characterized in that the second suspension contains at least 10-25times the weight of water, based on the weight of the binder.
 3. Amethod according to claim 1, wherein the first suspension is admixedwith an anticoagulant.
 4. A method according to claim 1, wherein themixture of the first and second suspensions is admixed with a releaseagent and/or an anti-foaming agent.
 5. A method according to claim 1,wherein the first suspension has LiAlO₂ introduced into it and theplate-shaped component comprises the matrix plate in an MCFC cell.
 6. Amethod according to claim 1, wherein the plate-shaped componentcomprises an electrode in an MCFC cell.
 7. A method according to claim6, comprising the cathode of an MCFC cell, to which agents are addedwhich retard dissolution of the cathode material.
 8. A method accordingto claim 1, wherein the component is used in an MCFC cell, and whereinthe second suspension is admixed with carbonate material in solution. 9.A method according to claim 8, wherein the carbonate material maycomprises carbonate salts or alkali metals in a mixing ratio.
 10. Amethod according to claim 8, wherein the second suspension contains atleast 10-25 times the weight of water, based on the weight of thebinder.
 11. A method according to claim 1, wherein the mixture of thefirst and second suspension is admixed with a plasticizer.
 12. A methodaccording to claim 1, wherein the alcohol comprises ethanol.
 13. Amethod according to claim 1, wherein fibres are added as a thirdsuspensions dispersed in an alcohol, the said third suspension beingadded to the first and second suspension.
 14. A method according toclaim 1, wherein the fibre material used comprises Al₂O₃ or LiAlO₂fibres.
 15. A method according to claim 1, wherein after mixing of thesuspensions and prior to tape casting the mixture obtained is screenedand de-aerated.
 16. A method according to claim 1, wherein the basematerial for the plates consists of production rejects.